sync: Added WeakSender to sync::broadcast::channel
#7100
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−1
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Darksonn
reviewed
Jan 27, 2025
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| /// Number of outstanding weak Sender handles. | ||
| num_weak_tx: AtomicUsize, |
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You are adding this only to add a weak_count method? You definitely do not need SeqCst for that.
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| pub fn is_closed(&self) -> bool { | ||
| // Channel is closed when there are no strong senders left active | ||
| self.shared.num_tx.load(Acquire) == 0 |
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Man, the orderings in this file are a mess. Mixing SeqCst and non-SeqCst orderings on the same atomic is not a good idea. The correct orderings for counters is:
- Increments happen with relaxed.
- Decrements happen with acqrel.
- Checks for zero happen with acquire.
We shouldn't need SeqCst for num_tx ever.
| @@ -56,6 +56,7 @@ macro_rules! assert_closed { | |||
| trait AssertSend: Send + Sync {} | |||
| impl AssertSend for broadcast::Sender<i32> {} | |||
| impl AssertSend for broadcast::Receiver<i32> {} | |||
| impl AssertSend for broadcast::WeakSender<i32> {} | |||
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Please also see the async_send_sync.rs file.
| match self | ||
| .shared | ||
| .num_tx | ||
| .compare_exchange_weak(tx_count, tx_count + 1, AcqRel, Acquire) |
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This is sufficient. When this succeeds, the operation is an increment for which relaxed is enough. When it fails, we might exit through the zero-check, so it needs acquire.
Suggested change
| .compare_exchange_weak(tx_count, tx_count + 1, AcqRel, Acquire) | |
| .compare_exchange_weak(tx_count, tx_count + 1, Relaxed, Acquire) |
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Motivation
Closing issue #7003. Add a type
WeakSenderto thesync::broadcast::channelsimilar tosync::maps::channel.Solution
The new
WeakSendertype just stores anArc<Shared<T>>just like the normalSenderbut activeWeakSenderswill not prevent the channel from being closed if allSenders are dropped.Closes #7003.